Wounding- and Elicitor-induced Formation of Coloured Chalcones and Flavans (as Phytoalexins) inHippeastrum x hortorum*

Transcriptional response of a novel HpCDPK1 kinase gene from Hippeastrum x hybr. to wounding and fungal infection

Calcium dependent protein kinases (CDPK) are well established plant sensor and effectors for calcium ions and participate in regulation of multiple abiotic and biotic stress responses in plant cells. Here we present the identification and characterization of a new CDPK kinase gene from bulbous plant Hippeastrum x hybr. and examine the role of this kinase in stress responses leading to phytoalexin (PA) production in plant tissues. In the previous research, it was shown that Hippeastrum bulbs mechanically wounded or infected with Peyronellaea curtisii (=Phoma narcissi) are inducted to an antifungal red substance synthesis. In this research, we demonstrated Ca²⁺ dependence of the phytoalexin production by wounded bulbs. Furthermore, the isolated HpCDPK1 cDNA for ORF was found to be 1596bp long and encoded 531 amino acid protein with CDPK kinase activity, as was shown by recombinant GST-HpCDPK1 enzyme production and analysis. HpCDPK1 transcript was present in all vegetative and chosen generative organs of Hippeastrum plant. The dynamics of the observed HpCDPK1 mRNA changes in bulbs depended on stressor type. The mechanical injury caused one wave of transcript increase while more complex transcript changes were observed within 48h after Peyronellaea inoculation. In plant bulbs already accumulating red phytoalexin, increases in HpCDPK1 mRNA level were observed at certain intervals within 48h whereas, in the case of fungal infection, only one big increment in the transcript amount at the 10th minute after inoculation was detected. The observed transcriptional response of HpCDPK1 gene to wounding and pathogen infection stress suggests a positive correlation with phytoalexin synthesis and maintenance in bulb tissues and puts more light on CDPK kinase role in the plant stress response regulation. This also bears some potential for understanding the mechanism of a phytoalexin formation.

Identification of a Hippeastrum hybridum guanylyl cyclase responsive to wounding and pathogen infection

Guanosine 3',5'-cyclic monophosphate (cGMP) is a critical component of many (patho)physiological processes in plants whilst guanylyl cyclases (GCs) which catalyse the formation of cGMP from GTP have remained somewhat elusive. Consequently, the two major aims are the discovery of novel guanylyl cyclases and the identification of GC/cGMP mediated processes. To identify a novel GC from Hippeastrum hybridum plant and facilitate the preparation of guanylyl cyclase in an amount sufficient for further crystallographic studies, we have constructed an overproduction system for this enzyme. This gene encodes a protein of 256 amino acids, with a calculated molecular mass of 28kD. The predicted amino acid sequence contains all the typical features and shows a high identity to other plant GCs. The GST-HpGC1 was catalytically active in Escherichia coli cells and the purified, recombinant HpGC1 was able to convert GTP to cGMP in the presence of divalent cations. The used overexpression system yields a guanylyl cyclase as 6% of the bacterial cytosolic protein. Besides the identification of HpGC1 as a guanylyl cyclase, the study has shown that the level of HpCG1 mRNA changed during stress conditions. Both mechanical damage and a Peyronellaea curtisii (=Phoma narcissi) fungi infection led to an initial decrease in the HpGC1 transcript level, followed by a substantial increase during the remainder of the 48-h test cycle. Moreover, significant changes in cyclic GMP level were observed, taking the form of oscillations. In conclusion, our data unequivocally identified the product of the HpGC1 gene as a guanylyl cyclase and demonstrates that such an overproduction system can be successfully used in enzyme synthesis. Furthermore, they indicate a link between the causing stimulus (wounding, infection) and guanylyl cyclase expression and the increase in cGMP amplitude. Therefore, it is concluded that appearance of cyclic GMP as a mediator in defense and wound-healing mechanisms provides a clue to the regulation of these processes.

Induction of stilbene phytoalexins in grapevine (Vitis vinifera) and transgenic stilbene synthase-apple plants (Malus domestica) by a culture filtrate of Aureobasidium pullulans

Products containing the epiphytic yeast Aureobasidium pullulans are commercially available and applied by fruit growers to prevent several fungal and bacterial diseases of fruit trees. The proposed beneficial mechanisms relate to limitations of space and nutrients for the pathogens in presence of the rapidly proliferating yeast cells. These explanations ignore the potential of yeasts to elicit the plant's defense. Our experiments aim at clarifying if an autoclaved and centrifuged suspension of A. pullulans may induce defense mechanisms. As a model system, the biosynthesis and accumulation of stilbene phytoalexins in callus and shoots of grapevine Vitis vinifera grown in vitro was used. Yeast application to the plant tissue stimulated stilbene biosynthesis, sometimes at the cost of flavonoids. The expression of the gene encoding stilbene synthase was enhanced and the enzyme showed higher activity while chalcone synthase activity and expression was reduced in some cases. An accumulation of stilbenes was also found in transgenic apple trees (Malus domestica cv. Holsteiner Cox) harboring the stilbene synthase-gene under control of its own promoter. These results clearly show that the application of A. pullulans may induce defense mechanisms of the treated plants.